Some scientists are skeptical about the detection of methane on Mars. Bruce Jakosky, a planetary geologist at the University of Colorado in Boulder, has studied atmospheric water on Mars for 25 years. He says he is not yet convinced that the methane measurements are real and statistically valid.

"Also, there is nothing in the water vapor measurements to suggest that there's a need to invoke a subsurface source," he said. "The distribution of water vapor is controlled primarily by the circulation of the atmosphere and the motions of the wind. If the water vapor and methane correlate, I would be more inclined to think the methane is also being redistributed by the wind."

Jakosky says there would have to be a "humongous" source of methane for the gas to be hovering over one region without being redistributed. "It's just not physically plausible," he said.

The evidence for an underground ice table around the Martian equator is also controversial. This is based on Odyssey's detection of hydrogen atoms in the top layer of the soil. Some experts interpret the hydrogen as being locked in ice and others say it could come from minerals affected by water in the past.

Formisano presented his results on September 20 at the International Mars Conference in Ischia, Italy. But the findings have not been published yet, which Jakosky says makes it difficult for the science community to evaluate.

"It's frustrating that we increasingly seem to be doing science by press release," Jakosky said. "It's time for these guys to publish their results."

Extinct Life?

Mars is the planet in our solar system that most closely resembles Earth. It has a rocky surface, perhaps making it easier for life to gain a foothold.

Earlier this year, NASA announced that its roving robot Opportunity had found evidence that water once soaked the planet Mars. Pictures from the rover's panoramic imager revealed salt-laden sediments that would have been shaped by flowing water or maybe a great Martian lake or sea.

Mars may not have only been wetter in the past, but could have had a denser atmosphere. There is the possibility that life arose on Mars, only to die out as conditions on the planet changed. Some researchers have suggested that future searches for life should focus on extinct, rather than current, life.